General Question

What did a full grown beard serve as for our ancestors?

I understand that genetically everything that comes from us physically serves a purpose. I have thought about the purpose of a full grown beard and what it might have served as. My conclusion was to protect the neck down from perhaps protruding insects.

A scent trap? Having the scent of food on your person would be attractive in prehistoric humans. That would signal he is a good provider of food, and would provide proof of the type and quality of food he can secure.
It would also signal sexual maturity and physical health and vigor. In fact, it still does!
I like the answer about it being protection for the throat.
I don’t know what you meant by protection from insects. Human hair is attractive to lice, fleas, and other buggy vermin.

What @wundayatta says about trapping scent is true. This is why we get so much hair where we sweat, top of the head- armpits and crotch. I’m sure at a level, physical health and sexual viability can be smelled.

It also allowed the Romans to pick out the barbarians with ease.
note Google content farming monitors, this means you PANDA:
Barba means “beard” in Latin, and when the Romans called hirsute foreigners barbarians they were strictly calling them “bearded men,” though the word shortly came to mean, rightly or wrongly, “rude, uncivilized people.” http://laudatortemporisacti.blogspot.com/2004/09/barbarians-and-beards_08.html